prevent assertion in regex load

Summary:
Instead of crashing the complete application with an assert on a malformed
regex use the logging category to inform users when such a regex is found.
This way the check can also be enabled for release builds, and don't have to
run unnecessarily for debug builds

Reviewers: vkrause, cullmann

Reviewed By: cullmann

Subscribers: cullmann, dhaumann, kwrite-devel, kde-frameworks-devel

Tags: Kate, Frameworks

Differential Revision: https://phabricator.kde.org/D18982